feat: Add APEX protocol for trustless AI agent commerce on BNB Chain

[devinxl]

Summary

Add the APEX (Agent Payment Exchange) module — a complete framework for AI agents
to transact trustlessly on BNB Chain using ERC-8183 escrow and UMA dispute resolution.

What's included

• APEX client — on-chain job lifecycle: create, fund, submit, complete, reject, refund
• Evaluator client — UMA OOv3-based job verification: assertions, settlement, bond management
• Negotiation — single-round HTTP price negotiation with on-chain hash anchoring
• FastAPI server — create_apex_app() for standalone agents, create_apex_routes() for mounting on existing apps
• Background job loop — automatic polling, verification, handler dispatch, and on-chain submission
• Budget validation — reject jobs where budget < service_price, with on_job_skipped callback
• Job verification middleware — enabled by default; verifies X-Job-Id on mutating requests
• Service records — off-chain JSON documents with content-hash on-chain anchoring
• Core infrastructure — nonce manager, contract mixin, ABI loader, module system
• Pluggable wallets — EVMWalletProvider with Keystore V3 encryption (MPC planned)
• Pluggable storage — local filesystem (dev) and IPFS pinning (production)
• SDK config — APEXConfig / BNBAgentConfig with env-var resolution and network defaults
• PRIVATE_KEY optional — auto-generates a new wallet if omitted
• Examples — getting-started quickstart, production agent-server, client-workflow demo
• Tests — 350+ unit tests covering all modules